Light emitting device having TFT

ABSTRACT

The present invention provides a TFT that has a channel length particularly longer than that of an existing one, specifically, several tens to several hundreds times longer than that of the existing one, and thereby allowing turning to an on-state at a gate voltage particularly higher than the existing one and driving, and allowing having a low channel conductance gd. According to the present invention, not only the simple dispersion of on-current but also the normalized dispersion thereof can be reduced, and other than the reduction of the dispersion between the individual TFTs, the dispersion of the OLEDs themselves and the dispersion due to the deterioration of the OLED can be reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacturing method ofsemiconductor device, in particular, present invention relates to alight emitting device comprising an organic light emitting device (OLED)formed over a substrate having an insulating surface. The invention alsorelates to an OLED module in which ICs including a controller, or thelike, is mounted with an OLED panel. Note that, in this specification,the light emitting device includes the OLED panel and for the OLEDmodule. Electronic equipment using the light emitting device is alsoincluded in the present invention.

Noted that in the present specification, the term “semiconductor device”generally indicates a device which is capable of functioning byutilizing semiconductor characteristics, and a light emitting device, anelectro-optical device, a semiconductor circuit and an electronic deviceare all included in the semiconductor device.

2. Description of the Related Art

Recently, technology for forming TFTs (Thin Film Transistor) over asubstrate has been greatly progressed, and its application to an activematrix display device is actively developed. In particular, a TFT usinga polysilicon film have a higher field effect mobility (also referred toas mobility) than that of a conventional TFT using an amorphous siliconfilm, and thus, is capable of high-speed operations. Therefore, adriving circuits that consist of TFTs using a polysilicon film isprovided over the same substrate as pixels, and the development forcontrolling respective pixels is performed actively. Since drivingcircuits and pixels over one substrate are incorporated into an activematrix display device, there are various advantages such as reduction inthe manufacturing cost, miniaturization of the display device,improvement in yield, and improvement in throughput.

In addition, an active matrix light emitting device (hereinafter, simplyreferred to as light emitting device), which has as a self-luminouselement an OLED is actively researched. The light emitting device isalso referred to as organic EL displays (OELDs) or organic lightemitting diodes (OLEDs).

An OLED is self-luminous to have high visibility, and is optimal formaking a display thin since a backlight like used for a liquid crystaldisplay (LCD) is not required. Further, an angle of view has no limits.Therefore, a light emitting device using an OLED has thus come under thespotlight as a substitute display device for CRTs and LCDs.

An active matrix driving system for displaying an image by arranging aplurality of TFTs in each pixel and sequentially writing a video signalis known as one mode of a light emitting device using OLED elements. TheTFT is an indispensable element for realizing the active matrix drivingsystem.

In addition, for the purpose of realizing the active matrix drivingsystem, in the light emitting device using OLED, since TFT controlscurrent amount flowing through OLED, it can not realized when TFT thatuses low current effect mobility amorphous silicon is adopted. It ispreferable that the semiconductor film having a crystallizing structure,typically, TFT using polysilicon is adopted to connect to OLED.

The semiconductor film having crystalline structure, typically, apolysilicon film is used to form TFT, and pixels and driving circuitsare formed integrally over the same substrate, thereby the number ofconnecting terminals is dramatically reduced and a frame area (theperiphery portion of the pixel portion) is also reduced.

However, even when the TFT is formed by using the polysilicon, itselectrical characteristics are finally not equivalent to thecharacteristics of a MOS transistor formed in a single crystallinesilicon substrate. For example, the electric field effect mobility of aconventional TFT is equal to or smaller than 1/10 in comparison with thesingle crystalline silicon. Further, the TFT using polysilicon has aproblem that is dispersion is caused easily in its characteristics dueto a defect formed in a boundary of a crystal grain.

In the light emitting device, at least a TFT functioning as a switchingelement and a TFT for supplying an electric current to an OLED aregenerally arranged in each pixel. A low off-electric current (I_(off))is required in the TFT functioning as the switching element while highdriving ability (an on-electric current I_(on)), the prevention ofdeterioration due to a hot carrier effect and the improvement ofreliability are required in the TFT for supplying the electric currentto the OLED. Further, high driving ability (the on-electric currentI_(on)), the prevention of deterioration due to the hot carrier effectand-the improvement of reliability are also required in the TFT of thedata line driving circuit.

Moreover, since the luminance of a pixel is determined by the ON current(I_(on)) of TFT which is electrically connected with an OLED andsupplies current to the EL element without depending on the drivemethod, there is a problem dispersion is caused in luminance if ONcurrent is not constant in case of displaying white on overall surface.For example, in case of adjusting luminance by light emitting time andperforming 64 gray scales, the ON current of the TFT which iselectrically connected with the EL element and supplies current to theOLED is dispersed 1.56% (= 1/64) from a fiducial point to shift one grayscale.

Moreover, when OLED is formed, the gap of EL layer pattering andunevenness of the thickness of EL layer disorder the substrate. There isa slightly variation in luminosity. This invention makes it the subjectto be made in view of the above-mentioned problem, to reduce thecharacteristic variation of each TFT, and to reduce the variation inluminosity.

Moreover, it is also making into the subject to reduce the variation inOLED which is not related to the characteristic variation of TFT, and toreduce the variation in luminosity.

SUMMARY OF THE INVENTION

Moreover, in conventional active matrix type light emitting device, whenresolution is tried to be raised, the problem that the aperture rate wasrestricted by arrangement of electrode for the retention capacitance ina pixel portion and the wiring for retention capacitance, TFT, variouswiring, and the like had occurred. This invention aims at offering thepixel structure which raises the aperture rate in a pixel portion.

As one of typical indicators of TFT characteristics, a V-Icharacteristics graph is known. At a position where a build-up in theV-I characteristics curve is most precipitous (it is also called as arising point), value of electric current changes most. Accordingly, inthe case that an electric current supplied to an OLED is controlled by aTFT, the-value of electric current of the TFT that supplies the currentto the OLED largely disperses when the rising point disperses.

The value of voltage at the rising point is called as threshold voltage(V_(th)) and is a voltage by which the TFT is switched to an on-state.Furthermore, in general, it is regarded that the closer to zero theV_(th) is, the better it is. It is regarded that when the V_(th) becomeslarger, an increase in a driving voltage and in power consumption may becaused.

There are two kinds of dispersions in the electric current value of theTFT. Specifically, one is the simple dispersion 3 sigma of the electriccurrent value and the other is the dispersion with respect to a mediumvalue (average value) of the electric current values in an ensemble of aparticular number of TFTs (in the present specification, this dispersionis also called as a normalized dispersion).

The present inventors have found that there is a tendency that thelatter dispersion depends strongly on gate voltage value (Vg). In FIG.3, relationship between Vgs in p-channel type TFTs (channel width W=8μm) of various channel lengths (5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200μm and 400 μm) and normalized dispersions is shown. Furthermore, in FIG.4, relationship between Vgs in n-channel type TFTs (channel width W=8μm) of various channel lengths and normalized dispersions is shown.

With experimental data of the TFTs, the present invention will now bedescribed in detail.

When the channel length of the TFT that supplies an electric current tothe OLED is made longer, the value of electric current becomes smallerand the simple dispersion 3 sigma decreases. TFTs are prepared with Vdset at −7V and Vg at −3.25 V and with the channel width fixed at 8 μm,and with the channel length varying in 50 μm, 100 μm, 200 μm and 400 μm,respectively. With each of the TFTs, the dispersion of on-current andnormalized dispersion are measured. These measurements are shown in FIG.11. However, as shown in FIG. 11, when only the channel length is madelonger, the current value becomes smaller, but the dispersion withrespect to the medium value of the electric currents in an ensemble of aparticular number of TFTs (normalized dispersion) does not change.

In the present invention, in order to make the dispersion lower, a TFTis designed to have such a long channel length as to be ten times ormore or several hundreds times-or more longer than ever so that the TFTmay be turned to an on-state at a particularly higher gate voltage, andfurthermore a gate voltage that is input from the outside is set todrive.

The TFTs whose Vd is set at −7V, channel width is fixed at 8 μm, andchannel length is set at 50 μm are measured of the dispersions of theon-currents and the normalized dispersions thereof at Vg=−3V,respectively. Subsequently, in a similar manner, the TFTs whose channellength is 100 μm are measured at Vg=−3.75 V, the TFTs whose channellength is 200 μm are measured at Vg=−3.75 V, and the TFTs whose channellength is 400 μm are measured at Vg=−5.75 V. Measurement results areshown in FIG. 2.

As shown in FIG. 2, as the channel length is made longer and thereby thegate voltage (Vg) is made larger, not only the simple dispersion ofon-current but also the normalized dispersion can be reduced. In thisexample, in order to make the Vg larger, the TFT having longer channellength is used. However, it is not restricted to the above but, in orderto make the Vg larger, within permissible design limits, for instance,the channel width W may be made shorter, a source region or a drainregion of the TFT may be made highly resistive, or a contact resistancemay be made higher.

Furthermore, the present invention provides a TFT whose channel lengthis much longer, specifically, several tens of to several hundreds oftimes longer than that of the related art, so that the TFT may be turnedto an on-state at a gate voltage much higher than ever to drive and mayhave low channel conductance gd. FIG. 1 shows data corresponding to FIG.2 and is a graph showing channel conductance gd of individual TFTs underthe same conditions (Vg, the channel width, and the channel length) asthe data of FIG. 2.

In the present invention, when a TFT that supplies a current to an OLEDis made such that in the range that the sum of source-drain voltage Vdand threshold voltage Vth is larger than gate voltage Vg, that is, inthe range of Vg<(Vd+Vth), channel conductance is from 0 to 1×10⁻⁸ S,preferably 5×10⁻⁹ S or less, further preferably 2×10⁻⁹ S or less so thatdispersion of the current that flows to the TFT can be reduced, and acertain constant current can be flowed to the OLED.

In addition to the above, resulting from the smaller channel conductancegd, the dispersion of the OLEDs themselves caused by an area contractionin an EL layer due to a patterning or heat treatment can be alsoreduced. Furthermore, by making the channel conductance gd smaller, evenwhen the OLED may be deteriorated for some reason, the current flowingto the OLED can be maintained at a constant value, resulting inmaintaining a constant brightness. In FIG. 12, Id-Vd curves and loadcurves of the OLED are shown. The channel conductance gd indicates agradient of the Id-Vd curve, and as the channel conductance gd is madesmaller, the gradient of the Id-Vd curve becomes smaller, resulting in asubstantially constant current value. In FIG. 12, the load curves of theOLED are curves showing relationship between the current value that isinput to the OLED and the Vd when Vg=−3.3 V and a p-channel TFTconnected to the OLED is driven in a saturation region. For instance,when −Vd is −17 V, since a voltage on a cathode side is −17 V, a voltagethat is input to the OLED is 0 V. Accordingly, a current that is inputto the OLED becomes also zero. Furthermore, the current value at anintersection point of the Id-Vd curve and the load curve of the OLEDcorresponds to the brightness. In FIG. 12, when the gd is smaller, thereis an intersection point where −Vd is −7 V. At that time, the currentvalue that is input to the OLED is 1×10⁻⁶ [A], and luminescence of thebrightness corresponding to this current value can be obtained. When thegd is smaller, to whichever side of a right side and a left side theload curve of the OLED may be moved, the current value hardly change,resulting in a uniform brightness. Furthermore, when an individual OLEDitself disperses, the load curve thereof moves to either the right sideor the left side. Furthermore, when the OLED deteriorates, the loadcurve of the OLED shifts to the left side. In the case that the gd islarger, when the load curve of the OLED shifts to the left side becauseof the deterioration and becomes a curve shown with a dotted line, anintersection point with the load curve of the OLED varies, resulting indifferent current values before and after the deterioration. On theother hand, in the case that the gd is smaller, even when the load curveof the OLED shifts to the left side because of the deterioration, thecurrent value hardly changes so that the dispersion of the brightness isreduced, resulting in a uniform brightness.

Here, in order to make the channel conductance gd lower, the channellength is made longer, and thereby the TFT is turned to an on-state at avoltage much higher than in the related art to be driven. However, byother means, the channel conductance gd may be further lowered. Forinstance, the channel conductance gd may be in lowered by forming theTFT in a LDD structure, or by dividing a channel forming region into aplurality of sub-regions.

Most n-channel TFTs of, pixels for use in liquid crystal panels are ofsize channel length L×channel width W=12 μm×4 μm and L×W=12 μm×6 μm. Ingeneral, in order to improve an open area ratio, it is regarded that thesmaller an area that the TFT occupies in a pixel, that is, an occupationarea, is, the better it is. Accordingly, it has been difficult to thinkof making the channel length such long as 100 μm or more. Furthermore,it is found that, as shown in FIG. 4, in the case that the channellength is 5 μm or 10 μm, the Vg least disperses in the range of 8 V to10 V and there is an increasing tendency in the dispersion when the Vgis 10 V or more. Accordingly, it could not be thought of that, in thecase that the channel length is made 100 μm or more, the larger the Vgis, the less the dispersion becomes.

Furthermore, when the channel length is made 100 μm or more, variousshapes can be thought of as a semiconductor layer. Typical examples,include a shape in which a semiconductor layer 102 snakes in an Xdirection as shown in FIG. 6 (it is referred to as A type in the presentspecification), a shape in which a semiconductor layer 1102 snakes in aY direction as shown in FIG. 13A (it is referred to as B type in thepresent specification), and a rectangular shape (a semiconductor layer1202) as shown in FIG. 13B.

Still furthermore, when the channel length is made longer, in the casethat a laser beam radiation process is applied as one of steps forforming the TFT, the dispersion of the laser beam can be also reduced.With each of combinations of the TFT sizes and the semiconductor layershapes of L×W=87 μm×7 μm (rectangular shape), L×W=165 μm×7 μm(rectangular shape), L×W=88 μm×4 μm (rectangular shape), L×W=165 μm×4 μm(rectangular shape), L×W=500 μm×4 μm (A type), and L×W=500 μm×4 μm (Btype), and furthermore with a scanning speed of the laser beam set at 1mm/sec or 0.5 mm/sec, the TFTs are prepared. With these TFTs,experiments are conducted to study the relationship between the TFT sizeand the shape of the semiconductor layer, and the dispersion (3 sigma)of the on-current of the TFT. Here, the laser beam is radiated toimprove the crystallinity of polysilicon. In FIG. 18, experimentalresults in the case of the gate voltage Vg=−5 V and Vd=−6 V are shown,and in FIG. 19, experimental results in the case of the gate voltageVg=−10 V and Vd=−6 V are shown. In FIGS. 18 and 19, medium values (μA)of the on-currents are also shown. Furthermore, relationship between theTFT size and the shape of the semiconductor layer, and the dispersion (3sigma) of the threshold value (Vth) of the TFT is obtained and shown inFIG. 20.

From FIGS. 18 and 19, it can be read that there is a tendency that thelonger the channel length L is, the smaller the dispersion of theon-currents is. The dispersion of the laser beam is smaller in the laserscanning speed 0.5 mm/sec than in 1 mm/sec, and the longer the channellength L is made, the smaller the difference of the dispersions of thedifferent laser scanning speeds becomes. That is, it can be regardedthat the longer the channel length L is made, the more the dispersion ofthe laser light can be reduced. Furthermore, it can be read that onewhose dispersion is most reduced is L×W=500 μm×4 μm, and the dispersionof the on-current is smaller in the A type than in the B type.

In view of the above, from FIGS. 18 and 19, it can be seen that thedispersion of the brightness of a light emitting device in which the TFTthat supplies a current to the OLED is operated in a voltage range untila saturation region is attained can be reduced.

Furthermore, when compared with the current value flowing to the TFTfixed at a constant value, the channel width W is better to be smaller.FIG. 21 shows a graph showing the dispersions when the current valuesare fixed at a constant value (Id=0.5 μA). From FIG. 21, it can be seenthat the dispersion of the brightness of the light emitting-device inwhich the TFT that supplies the current to the OLED is operated in thesaturation region can be reduced. Furthermore, similarly, it can be readthat the dispersion is most reduced in L×W=500 μm×4 μm, and theon-current of the A type disperses less than the B type.

Still furthermore, FIG. 20 also tells that there is a tendency that thelonger the channel length L is, the less the dispersion of the thresholdvoltage (Vth) is.

Furthermore, since as the channel length L is made longer, thedispersions of both the threshold values and on-currents, that is,electric characteristics of the TFT, are reduced, it can be regardedthat not only the dispersion of the laser beam is reduced but also thedispersion resulting from other processes is reduced.

Still furthermore, also in a light emitting device having an OLED, it isregarded that the smaller the occupation area of the TFT that isprovided to the pixel is, the better the TFT is. Since the existing TFTsize is small, the dispersion in the individual TFT characteristics islarge and is a main reason of the display irregularity in a displaydevice.

In the case of the current flowing to the OLED being controlled with theTFT, largely divided, there are two methods. Specifically, one is amethod that controls the current in a voltage range called thesaturation region and the other one is a method that controls thecurrent in the voltage range until the saturation region is attained.When, as shown in FIG. 9, with a certain constant gate voltage appliedand with a source-drain voltage Vd gradually raising, current valuesflowing between the source and the drain are measured, and thereby aVd−Id curve of a TFT is obtained, a graph in which the current valuebecomes a substantially constant above a certain value of Vd isobtained. In the present specification, in the Vd−Id curve, a rangewhere the current value becomes substantially constant is called asaturation region.

The present invention is also effective even when the TFT that suppliesthe current to the OLED is operated in the voltage range until thesaturation region is attained. However, in particular, when the TFT thatsupplies the current to the OLED is operated in the saturation regionand thereby the current flowing to the OLED is maintained constant, aneffect of reducing the dispersion is remarkable.

Furthermore, it is preferable to use the p-channel type TFT whosedispersion is more reduced than the n-channel type TFT, as shown inFIGS. 3 and 4, for the TFT that supplies the current to the OLED.However, in the present invention, the TFT that supplies the current tothe OLED may be either one of the n-channel type TFT and the p-channeltype TFT. In the case of the TFT that supplies the current to the OLEDbeing, for instance, the p-channel type TFT, a connection need only beperformed as shown in FIG. 10A. Furthermore, in the case of the TFT thatsupplies the current to the OLED being, for instance, the n-channel typeTFT, a connection need only be implemented as shown in FIG. 10B. In eachof FIGS. 10A and 10B, although only the TFT that supplies the current tothe OLED is shown, it goes without saying that after the gate electrodeof the TFT, various circuits made of a plurality of TFTs may bedisposed. That is, the circuit configuration is not restricted toparticular one.

One configuration of the invention that is disclosed in the presentspecification is a light emitting device having a light emittingelement, the light emitting element including:

-   -   a cathode;    -   an organic compound layer in contact with the cathode; and    -   an anode in contact with the organic compound layer;    -   wherein a channel length L of a TFT connected to the light        emitting element is 100 μm or more, and preferably is from 100        μm to 500 μm.

In the configuration, a ratio of a channel width W of the TFT to thechannel length L thereof is from 0.1 to 0.01.

Another configuration of the invention that is disclosed in the presentspecification is a light emitting device having a light emittingelement, the light emitting element, including:

-   -   a cathode;    -   an organic compound layer in contact with the cathode; and    -   an anode in contact with the organic compound layer;    -   wherein a ratio of a channel width W of the TFT connected to the        light emitting element to the channel length L thereof is from        0.1 to 0.01.

In the respective configurations, the TFT connected to the lightemitting element, in the range that the sum of source-drain voltage Vdand threshold voltage Vth is larger than gate voltage Vg, has a channelconductance gd from 0 to 1×10⁻⁸ S, preferably of 0 to 5×10⁻⁹ S, morepreferably of 0 to 2×10⁹ S.

Still another configuration of the invention that is disclosed in thepresent specification is a light emitting device having a light emittingelement, the light emitting element, including:

-   -   a cathode;    -   an organic compound layer in contact with the cathode; and    -   an anode in contact with the organic compound layer;    -   wherein the TFT connected to the light emitting element, in the        range that the sum of source-drain voltage Vd and threshold        voltage Vth is larger than gate voltage Vg, has a channel        conductance gd from 0 to 2×10⁻⁹ S.

In the respective configurations, the TFT connected to the lightemitting element is a p-channel type TFT or an n-channel type TFT.

A region that is called a channel region in the present specificationdenotes a region that contains a portion (it is called also a channel)where carriers (electrons and holes) flow, and a length of the channelregion in a direction in which the carriers flow is called a channellength and a width thereof a channel width.

Furthermore, in the specification, the channel conductance gd denotes aconductivity of a channel and can be expressed with the followingequation.gd=W(V _(g) −V _(th))μ_(n) C _(ox) /L [Equation 1]

In the equation 1, L denotes a channel length, W a channel width, Vg agate voltage, Vth a threshold voltage, μn mobility, and C_(ox) an oxidefilm capacitance. In the TFT, when the Vg is equal to or more than theVth, the channel conductance starts to generate.

In addition to this, in the case of the channel length L being madelonger, the oxide film capacitance C_(ox) becomes larger. Accordingly,the capacitance can be partially made use of as a retention capacitanceof the OLED. So far, in order to form a retention capacitance, a spacefor forming the retention capacitance is necessary for each of pixels,and a capacitance line and a capacitance electrode are disposed.However, when a pixel configuration of the present invention is adopted,the capacitance line and the capacitance electrode can be omitted.Furthermore, in the case of the retention capacitance being formed withthe oxide film capacitance C_(ox) , the retention capacitance can beformed, with a gate insulating film as dielectrics, of a gate electrodeand a semiconductor (channel region) that overlaps with the gateelectrode with the gate insulating film interposed therebetween.Accordingly, even in the case of the channel length of the TFT beingmade longer, as shown in FIG. 5, when a semiconductor layer 102 of theTFT is disposed below a power supply line 106 disposed at an upper layerof the gate electrode and a source wiring, a pixel can be designedwithout decreasing the open area ratio. That is, when the present pixelconfiguration is implemented, even when the space for the capacitanceline and the capacitance electrode is omitted, sufficient retentioncapacitance can be provided, and furthermore the open area ratio can beimproved.

In the combinations of the TFT sizes and the semiconductor layer shapesshown in FIGS. 18 and 19, the oxide film capacitances C_(ox) are 192(fF) for L×W=87 μm×7 μm (rectangular shape) case, 364.5 (fF) for L×W=165μm×7 μm (rectangular shape) case, 111.1 (fF) for L×W=88 μm×4 μm(rectangular shape) case, 208.3 (fF) for L×W=165 μm×4 μm (rectangularshape) case, 631.3 (fF) for L×W =500 μm×4 μm (A type) case, and 631.3(fF) for L×W=500 μm×4 μm (B type) case, respectively. Furthermore, othervalues when the oxide film capacitance C_(ox) is obtained are set asfollows. That is, a film thickness of the gate insulating film (oxidefilm) Tox is 115 nm, ε_(o) is 8.8542×10⁻¹² (F/m²), and ε_(ox) is 4.1.

Furthermore, in the respective configurations, the capacitance C_(ox) ofthe TFT connected to the light emitting element is 100 fF or more, beingpreferably in the range of 100 fF to 700 fF.

Still furthermore, in the respective configurations, the gate electrodeof the TFT connected to the light emitting element and a wiring disposedthereabove form a retention capacitance. Specifically, as shown in FIG.5, with an interlayer insulating film (an organic insulating film orinorganic insulating film) disposed on the gate electrode 100 asdielectrics, the gate electrode 100 and a power supply line 106 thatoverlaps with the gate electrode 100 form a capacitance. In FIG. 5, anarea with which the gate electrode 100 and the power supply line 106overlap (12 μm×127 μm=about 1524 μm²) is large, though depending on thefilm thickness and dielectric constant of the interlayer insulatingfilm, a retention capacitance is formed. All of the capacitance formedbetween the gate electrode 100 and the power supply line 106 is allowedto function as a retention capacitance of an EL element. Accordingly, itis preferable to appropriately design so that the sum of the capacitanceC_(ox) of the TFT that is connected to the light emitting element andthe capacitance that is formed between the gate electrode of the TFT andthe power supply line 106 may be several hundreds fF.

In the present specification, all layers formed between an anode and acathode of the OLED are defined as an organic light emitting layers. Theorganic light emitting layers, specifically, comprises a light emittinglayer, a hole injection layer, an electron injection layer, a holetransporting layer, and an electron transporting layer. Basically, theOLED has a structure in which an anode, a light emitting layer and acathode are sequentially stacked. In addition to this structure, thereare other structures in which an anode, a hole injection layer, a lightemitting layer, and a cathode are sequentially stacked, or an anode, ahole injection layer, a light emitting layer, an electron transportinglayer, and a cathode are sequentially stacked.

An OLED includes a layer that contains an organic compound (organiclight emitting material) from which luminescence (Electro-luminescence)can be obtained when an electric field is applied (hereinafter, referredto as organic light emitting layer), an anode and a cathode. In theluminescence in the organic compound, there are luminescence generatedwhen an excited singlet state relaxes to a ground state (fluorescence)and luminescence generated when an excited triplet state relaxes to theground state (phosphorescence). In the light emitting device of thepresent invention, among the above luminescences, either one of theabove luminescences may be used, or both of the luminescences may beused.

Furthermore, in the above, as an illustration, a top gate type TFT isexplained. However, the present invention can be applied withoutrestricting to a particular TFT structure. The present invention can beapplied to, for instance, a bottom gate type (inverse stagger type) TFTand a forward stagger type TFT.

Still furthermore, in the light emitting device of the presentinvention, a driving method for displaying a screen is not restricted toa particular method. For instance, a dot sequential driving method, aline sequential driving method or a plane sequential driving method canbe used. Typically, with the line sequential driving method, atime-sharing gradation driving method or an area gradation drivingmethod may be appropriately applied. Furthermore, a video signal that isinput to a source line of the light emitting device may be an analogsignal or a digital signal, a driving circuit or the like beingappropriately designed in accordance with a video signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing relationship between channel length of a TFTand channel conductance gd.

FIG. 2 is a diagram showing three sigma showing dispersion of currentand three sigma showing normalized dispersion of current.

FIG. 3 is a diagram showing relationship between dispersion of currentof p-channel type TFT and Vg at certain channel lengths.

FIG. 4 is a diagram showing relationship between dispersion of currentof n-channel type TFT and Vg at certain channel lengths.

FIG. 5 is a diagram showing a top view of a pixel.

FIG. 6 is a diagram showing a top view of a pixel.

FIG. 7 is a diagram showing a sectional structure of an active matrixtype light emitting display device.

FIG. 8 is a diagram showing an equivalent circuit of an active matrixtype light emitting display device.

FIG. 9 is a diagram showing a graph showing an Id-Vd curve.

FIGS. 10A and 10B are diagrams showing connection relations between anOLED and a TFT connected to the OLED.

FIG. 11 is a diagram showing three sigma showing the dispersion ofcurrent and three sigma showing the normalized dispersion of current.

FIG. 12 is a diagram showing a load curve and an Id-Vd curve of theOLED.

FIGS. 13A and 13B are diagrams showing top views of pixels (Embodiment2).

FIGS. 14A and 14B are diagrams showing a module (Embodiment 3).

FIG. 15 is a diagram showing a module (Embodiment 3).

FIGS. 16A through 16F are diagrams showing electronics (Embodiment 4).

FIGS. 17A through 17C are diagrams showing electronics (Embodiment 4).

FIG. 18 is a diagram showing relationship between TFT size of thepresent invention and the dispersion of on-current (at Vg=−5 V).

FIG. 19 is a diagram showing relationship between TFT size of thepresent invention and the dispersion of on-current (at Vg=−10 V).

FIG. 20 is a diagram showing relationship between TFT size of thepresent invention and the dispersion of threshold voltage.

FIG. 21 is a diagram showing relationship between TFT size of thepresent invention and the dispersion of on-current at a constant currentvalue (Id=0.5 μA).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, modes for implementing the present invention will beexplained.

FIG. 5 is a partially enlarged top view of a pixel portion of a lightemitting device having an OLED. In FIG. 5, for the sake of simplicity,an EL layer is not shown, and only one electrode (pixel electrode 107)of the OLED is shown.

In FIG. 5, a semiconductor layer 101 is a layer that works as an activelayer of a switching TFT, a region that overlaps with a gate wiring 105is a channel forming region, a region that connects with a source wiring104 is a source region (or a drain region), and a region that connectswith a connection electrode 103 is a drain region (or a source region).The switching TFT is a double-gate structure having two channel formingregions.

Furthermore, a semiconductor layer 102 is a layer that works as anactive layer of a TFT that supplies a current to the OLED, a region thatoverlaps with a gate electrode 100 being a channel forming region. Thegate electrode 100 of the TFT that supplies a current to the OLED isconnected with the connection electrode 103. Still furthermore, a sourceregion (or a drain region) of the TFT that supplies a current to theOLED and a power supply line 106 are connected, a drain region (or asource region) of the TFT that supplies a current to the OLED and aconnection electrode 108 being connected, and a pixel electrode 107being formed in contact with the connection electrode 108. Furthermore,above the gate electrode 100, the power supply line 106 and a sourcewiring of an adjacent pixel are disposed so as to partially overlap. Ofthe semiconductor layer 102, above a channel forming region thatoverlaps with the gate electrode 100 with the gate insulating filminterposed therebetween, the power supply line 106 and a source wiringof an adjacent pixel are disposed so as to partially overlap. All of thecapacitance formed between the gate electrode 100 and the power supplyline 106 can be used as a retention capacitance of the EL element.Accordingly, with the capacitance formed between the gate electrode 100and the power supply line 106, necessary retention capacitance can besecured to a certain degree.

Furthermore, FIG. 6 is a top view corresponding to FIG. 5 and is adiagram at a stage where the semiconductor layers 101 and 102, the gatewiring 105 and the gate electrode 100 are formed. A region where thesemiconductor layer 102 overlaps with the gate electrode 100 with a gateinsulating film (not shown) interposed therebetween, that is, a channelforming region is shown with a dotted line in FIG. 6.

The present invention intends to provide a TFT that supplies a currentto an OLED, and in the TFT, a length of a channel region (channel lengthL) is made particularly longer (L=100 to 500 μm, in this case 500 μm isadopted), and thereby the TFT is allowed turning to an on-state at agate voltage particularly higher than ever and driving, a channelconductance gd thereof being smaller (gd=0 to 1×10⁻⁸ S, preferably5×10⁻⁹ S or less, and in this case 2×10⁻⁹ S or less).

By taking the above configuration, as shown in FIG. 2, in a pixelportion where a plurality of TFTs are arranged, in the TFTs that supplycurrents to the OLED, not only the simple dispersion of on-current butalso the normalized dispersion thereof can be reduced, resulting inparticularly reducing the dispersion of brightness of a display devicehaving the OLED.

Furthermore, when as a driving method of the OLED a method in which acurrent flowing to the OLED is controlled in a voltage range called asaturation region is adopted, the present invention exhibits anextremely conspicuous effect. When the configuration is adopted, asshown in FIG. 12, other than the reduction of the dispersion between theindividual TFTs, also the dispersion caused at the preparation of theOLED (dispersion of the OLED itself caused by an area contraction of theEL layer at the patterning and heat treatment) can be reduced.Furthermore, by adopting the configuration, as shown in FIG. 12, otherthan the reduction of the dispersion between the individual TFTs, evenwhen the OLED is deteriorated for some reason, the current flowing tothe OLED can be maintained constant, resulting in maintaining a constantbrightness.

Still furthermore, in the present invention, as a method of driving theOLED, a method that controls the current flowing to the OLED in avoltage region until the saturation region is attained is also useful.

It goes without saying that the present invention is not restricted tothe top views shown in FIGS. 5 and 6. In FIGS. 5 and 6, a light emittingdevice that allows emitting light through a substrate over which the TFTis formed (the light emitting device shown in FIG. 14 is typical one) isillustrated. Accordingly, of the pixel electrode 107, an open areaportion is a region where the connection electrode 108 is not formed,and in order to make the open area portion larger, a TFT whose channellength L is long is disposed below the power supply line 106 and thesource wiring. All of capacitance formed between the gate electrode 100of the TFT whose channel length L is long and the power supply line 106can be used as the retention capacitance of the EL element. Furthermore,in the case of a light emitting device that emits light in a directionopposite to FIGS. 5 and 6 (a light emitting device shown in FIG. 15 istypical one), the open area portion becomes the same region as the pixelelectrode. Accordingly, the TFT whose channel length L is long may bedisposed below the pixel electrode, and a TFT having a further longerchannel length L of 500 μm or more can be formed.

Furthermore, when the pixel structure shown in FIGS. 5 and 6 is adopted,without forming a capacitance portion for the formation of the retentioncapacitance, the oxide film capacitance C_(ox) can be partially used asthe retention capacitance. However, in one pixel, the retentioncapacitance and a memory (SRAM, DRAM or the like) may be formed. Stillfurthermore, in one pixel, a plurality of TFTs (two or more TFTs) andvarious circuits (current mirror circuit or the like) may beincorporated.

Furthermore, although in the above a top gate type TFT is illustrated,irrespective of the TFT structures, the present invention can beapplied. The present invention can be applied to, for instance, a bottomgate type (inverse stagger type) TFT and a forward stagger type TFT.

The present invention thus configured will be detailed with reference tothe following embodiments.

Preferred Embodiments Embodiment 1

Here, a method of simultaneously manufacturing a pixel portion and TFTs(n-channel TFTs and a p-channel TFT) of a driving circuit provided inthe periphery of the pixel portion over the same substrate tomanufacture the light emitting device having OLED is described indetail.

For a lower layer of the base insulating film 301, a silicon oxynitridefilm formed from SiH₄, NH₃, and N₂O as material gases (compositionratio: Si=32%, O=27%, N=24%, H=17%) is formed on the heat resistanceglass substrate (the first substrate 300) having a thickness of 0.7 mmwith a thickness of 50 nm (preferably 10 to 200 nm) and at a filmdeposition temperature of 400° C. by using plasma CVD. Then, after thesurface is cleaned with ozone water, an oxide film on the surface isremoved by means of dilute hydrofluoric acid (dilution with 1/100).Next, for an upper layer of a base insulating film 302, a siliconhydride oxynitride film formed from SiH₄ and N₂O as material gases(composition ratio: Si=32%, O=59%, N=7%, H=2%) is formed thereon with athickness of 100 nm (preferably 50 to 200 nm) and at a film depositiontemperature of 400° C. by using plasma CVD to thereby form a lamination.Further, without exposure to an atmosphere, a semiconductor film havingan amorphous structure (in this case, amorphous silicon film) is formedto have a thickness of 54 nm (preferably 25 to 80 nm) with SiH₄ as afilm deposition gas and at a film deposition temperature of 300° C. byusing plasma CVD.

In this embodiment, the base insulating film 104 is shown in a form of atwo-layer structure, but a single layer of the insulating film or astructure in which two or more layers thereof are laminated may beadopted. Further, there is no limitation on the material of thesemiconductor film. However, the semiconductor film may be preferablyformed of silicon or silicon germanium (Si_(1-x)Ge_(x) (X=0.0001 to0.02)) alloy by using a known means (sputtering, LPCVD, plasma CVD, orthe like). Further, a plasma CVD apparatus may be a single wafer typeone or a batch type one. In addition, the base insulating film and thesemiconductor film may be continuously formed in the same film formationchamber without exposure to an atmosphere.

Subsequently, after the surface of the semiconductor film having anamorphous structure is cleaned, an extremely thin oxide film with athickness of about 2 nm is formed from ozone water on the surface. Then,in order to control a threshold value of a TFT, doping of a minuteamount of impurity element (boron or phosphorous) is performed. Here, anion doping method is used in which diborane (B₂H₆) is plasma-excitedwithout mass-separation, and boron is added to the amorphous siliconfilm under the doping conditions: an acceleration voltage of 15 kV; agas flow rate of diborane diluted to 1% with hydrogen of 30 sccm; and adosage of 2×10¹²/cm². Then, a nickel acetate salt solution containingnickel of 10 ppm in weight is applied using a spinner. Instead of theapplication, a method of spraying nickel elements to the entire surfaceby sputtering may also be used.

Then, heat treatment is conducted to perform crystallization, therebyforming a semiconductor film having a crystalline structure. A heatingprocess using an electric furnace or irradiation of strong light may beconducted for this heat treatment. In case of the heating process usingan electric furnace, it may be conducted at 500 to 650° C. for 4 to 24hours. Here, after the heating process (500° C. for 1 hour) fordehydrogenation is conducted, the heating process (550° C. for 4 hours)for crystallization is conducted, thereby obtaining a silicon filmhaving a crystalline structure. Note that, although crystallization isperformed by using the heating process using a furnace, crystallizationmay be performed by means of a lamp annealing apparatus. Also note that,although a crystallization technique using nickel as a metal elementthat promotes crystallization of silicon is used here, other knowncrystallization techniques, for example, a solid-phase growth method anda laser crystallization method, may be used.

Next, after the oxide film on the surface of the silicon film having acrystalline structure is removed by dilute hydrofluoric acid or thelike, irradiation of first laser light (XeCl: wavelength of 308 nm) forraising a crystallization rate and repairing defects remaining incrystal grains is performed in an atmosphere or in an oxygen atmosphere.Excimer laser light with a wavelength of 400 nm or less, or secondharmonic wave or third harmonic wave of a YAG laser is used for thelaser light. In any case, pulse laser light with a repetition frequencyof approximately 10 to 1000 Hz is used, the pulse laser light iscondensed to 100 to 500 mJ/cm² by an optical system, and irradiation isperformed with an overlap ratio of 90 to 95%, whereby the silicon filmsurface may be scanned. Here, the irradiation of the first laser lightis performed in an atmosphere with a repetition frequency of 30 Hz andenergy density of 470 mJ/cm². Note that an oxide film is formed on thesurface by the first laser light irradiation since the irradiation isconducted in an atmosphere or in an oxygen atmosphere. Though an exampleof using the pulse laser is shown here, the continuous oscillation lasermay also be used. When a crystallization of an amorphous semiconductorfilm is conducted, it is preferable that the second harmonic through thefourth harmonic of basic waves is applied by using the solid state laserwhich is capable of continuous oscillation in order to obtain a crystalin large grain size. Typically, it is preferable that the secondharmonic (with a thickness of 532 nm) or the third harmonic (with athickness of 355 nm) of an Nd: YVO₄ laser (basic wave of 1064 nm) isapplied. Specifically, laser beams emitted from the continuousoscillation type YVO₄ laser with 10 W output is converted into aharmonic by using the non-linear optical elements. Also, a method ofemitting a harmonic by applying crystal of YVO₄ and the non-linearoptical elements into a resonator. Then, more preferably, the laserbeams are formed so as to have a rectangular shape or an ellipticalshape by an optical system, thereby irradiating a substance to betreated. At this time, the energy density of approximately 0.01 to 100MW/cm² (preferably 0.01 to 10 MW/cm²) is required. The semiconductorfilm is moved at approximately 10 to 2000 cm/s rate relativelycorresponding to the laser beams so as to irradiate the semiconductorfilm.

Though the technique of irradiating laser light is conducted after heattreatment using nickel as a metal element for promoting thecrystallization is performed here, crystallization of an amorphoussilicon film may be performed by using continuous oscillation layer (thesecond harmonics of YVO₄ laser) without doping nickel.

The oxide film formed by this laser light irradiation and an oxide filmformed by treating the surface with ozone water for 120 seconds togethermake a barrier layer that has a thickness of 1 to 5 nm in total. Thoughthe barrier layer is formed by using ozone water here, another methodsuch as ultraviolet light irradiation performing in an oxygen atmosphereor oxide plasma treatment to oxidize the surface of the semiconductorfilm having the crystalline structure may be used. In addition, asanother method for forming the barrier layer, an oxide film having athickness of about 1 nm to 10 nm may be deposited by a plasma CVDmethod, a sputtering method, an evaporation method, or the like. In thisspecification, the term barrier layer refers to a layer which has a filmquality or film thickness that allows a metal element to pass in thegettering step and which functions as an etching stopper in the step ofremoving the layer that functions as a gettering site.

On the barrier layer, an amorphous silicon film containing argonelements are formed to a thickness of 50 to 400 nm, in this embodiment,150 nm by sputtering to serve as a gettering site. Film formationconditions by sputtering in this embodiment include setting the filmformation pressure to 0.3 Pa, the gas (Ar) flow rate to 50 sccm, thefilm formation power to 3 kW, and the substrate temperature to 150° C.The amorphous silicon film that is formed under the above conditionscontains argon elements in an atomic concentration of 3×10²⁰ to6×10²⁰/cm³, and contains oxygen in an atomic concentration of 1×10¹⁹ to3×10¹⁹/cm³. Thereafter, an electric furnace is used in heat treatment at550° C. for 4 hours for gettering to reduce the nickel concentration inthe semiconductor film having a crystalline structure. The lampannealing apparatus may by used instead of the electric furnace.

Subsequently, the amorphous silicon film containing the argon element,which is the gettering site, is selectively removed with the barrierlayer as an etching stopper, and then, the barrier layer is selectivelyremoved by dilute hydrofluoric acid. Note that there is a tendency thatnickel is likely to move to a region with a high oxygen concentration ingettering, and thus, it is desirable that the barrier layer comprised ofthe oxide film is removed after gettering.

Then, after a thin oxide film is formed from ozone water on the surfaceof the obtained silicon film having a crystalline structure (alsoreferred to as polysilicon film), a mask made of resist is formed, andan etching process is conducted thereto to obtain a desired shape,thereby forming the island-like semiconductor layers separated from oneanother. After the formation of the semiconductor layers, the mask madeof resist is removed.

Then, the oxide film is removed with the etchant containing hydrofluoricacid, and at the same time, the surface of the silicon film is cleaned.Thereafter, an insulating film containing silicon as its mainconstituent, which becomes a gate insulating film 303, is formed. Inthis embodiment, a silicon oxynitride film (composition ratio: Si=32%,O=59%, N=7%, H=2%) is formed with a thickness of 115 nm by plasma CVD.

Next, on the gate insulating film 303, a first conductive film with athickness of 20 to 100 nm and a second conductive film with a thicknessof 100 to 400 nm are formed in lamination. In this embodiment, a 50 nmthick tantalum nitride film and a 370 nm thick tungsten film aresequentially laminated on the gate insulating film 303.

As a conductive material for forming the first conductive film and thesecond conductive film, an element selected from the group consisting ofTa, W, Ti, Mo, Al and Cu, or an alloy material or compound materialcontaining the above element as its main constituent is employed.Further, a semiconductor film typified by a polycrystalline silicon filmdoped with an impurity element such as phosphorous, or an AgPdCu alloymay be used as the first conductive film and the second conductive film.Further, the present invention is not limited to a two-layer structure.For example, a three-layer structure may be adopted in which a 50 nmthick tungsten film, an alloy film of aluminum and silicon (Al—Si) witha thickness of 500 nm, and a 30 nm thick titanium nitride film aresequentially laminated. Moreover, in case of a three-layer structure,tungsten nitride may be used in place of tungsten of the firstconductive film, an alloy film of aluminum and titanium (Al—Ti) may beused in place of the alloy film of aluminum and silicon (Al—Si) of thesecond conductive film, and a titanium film may be used in place of thetitanium nitride film of the third conductive film. In addition, asingle layer structure may also be adopted.

An ICP (inductively coupled plasma) etching method may be preferablyused for the etching process of the above-mentioned first and secondconductive films (the first and second etching processes). The ICPetching method is used, and the etching conditions (an electric energyapplied to a coil-shape electrode, an electric energy applied to anelectrode on a substrate side, a temperature of the electrode on thesubstrate side, and the like) are appropriately adjusted, whereby a filmcan be etched to have a desired taper shape. In this embodiment, afterthe resist mask is formed, RF (13.56 MHz) power of 700 W is applied tothe coil-shape electrode with a pressure of 1 Pa as a first etchingcondition, and CF₄, SF₆, and NF₃, and O₂ can be appropriately used asetching gases. Each flow rate of gasses is set to 25/25/10 (sccm), andRF (13.56 MHz) power of 150 W is applied also to the substrate (samplestage) to substantially apply a negative self-bias voltage. Note that,size of the electrode area on the substrate side is 12.5 cm×12.5 cm, andcoil-shape electrode (a quartz disc comprising a coil is used here) has25 cm in diameter. With the first etching conditions, a W film is etchedto form an end portion of the first conductive layer into a taperedshape. Thereafter, the resist mask is removed and the second etchingcondition is adopted. CF₄ and Cl₂ are used as etching gases, the flowrate of the gases is set to 30/30 sccm, and RF (13.56 MHz) power of 500W is applied to a coil-shape electrode with a pressure of 1 Pa togenerate plasma, thereby performing etching for about 30 seconds. RF(13.56 MHz) power of 20 W is also applied to the substrate side (samplestage) to substantially apply a negative self-bias voltage. Under thesecond etching conditions in which CF₄ and Cl₂ are mixed, both the Wfilm and the TaN film are etched at the same level. Here, the firstetching condition and the second etching condition are referred to asthe first etching treatment.

The second etching treatment is performed without removing a resistmask. Here, CF₄ and Cl₂ are used as etching gases, the flow rate of thegases is set to 30/30 sccm, and RF (13.56 MHz) power of 500 W is appliedto a coil-shape electrode with a pressure of 1 Pa to generate plasma,thereby performing etching for about 60 seconds. RF (13.56 MHz) power of20 W is also applied to the substrate side (sample stage) tosubstantially apply a negative self-bias voltage. Thereafter, the fourthetching treatment is performed without removing a resist mask, CF₄, Cl₂,and O₂ are used as etching gases, the flow rate of the gases is set to20/20/20 sccm, and RF (13.56 MHz) power of 500 W is applied to acoil-shape electrode with a pressure of 1 Pa to generate plasma, therebyperforming etching for about 20 seconds. RF (13.56 MHz) power of 20 W isalso applied to the substrate side (sample stage) to substantially applya negative self-bias voltage. Here, the third etching condition and thefourth etching condition are referred to as the second etchingtreatment. At this stage, the gate electrode and electrodes 304 and 305to 307 comprised of the first conductive layer 304 a as a lower layerand the second conductive layer 304 b as a upper layer are formed. Atthis state, the upper structure of pixels may be formed as shown in FIG.6.

After removing the resist masks, the first doping treatment is conductedto dope using gate electrodes 304 to 307 as masks to entire surface. Thefirst doping treatment employs ion doping or ion implantation. In iondoping, the dose is set to 1.5×10¹⁴ atoms/cm² and the accelerationvoltage is set to 60 to 100 keV. Typically, phosphorus (P) or arsenic(As) is used as an impurity element that gives the n-type conductivity.The first impurity regions (n⁻ region) 322 to 325 are formed in a selfaligning manner.

Subsequently, new resist masks are formed. The masks are formed to coverthe channel formation region or the portion of the semiconductor layerfor forming the switching TFT 403 of the pixel portion 401. The masksare formed to protect the channel formation region or the portion of thesemiconductor layer for forming the p-channel TFT 406 of the drivingcircuit. In addition, masks are formed to cover the channel formationregion of the semiconductor layer for forming the current control TFT404 of the pixel portion 401 or the periphery portion thereof.

Next, the impurity region (n⁻ region) overlapping with a part of thegate electrode by performing selectively the second doping treatmentusing resist masks. The second doping processing may be performed by theion-doping method or the ion-implanting method. In this embodiment, theion doping method is performed under a condition in a gas flow rate ofphosphine (PH₃) diluted to 5% with hydrogen of 30 sccm, and the dose of1.5×10¹³ atoms/cm² and the accelerating voltage of 90 kV. The resistmask and the second conductive film function as mask for the n-typedoping impurity element, and the second impurity regions 311 and 312 areformed. An n-type doping impurity element in the density range of 1×10¹⁶to 1×10¹⁷ atoms/cm³ are added to the impurity regions 311 and 312. Inthis embodiment, the region of same concentration range as the secondimpurity region is referred to as n⁻ region.

The third doping processing is performed without removing masks made ofresist. The third doping processing may be performed by the ion-dopingmethod or the ion-implanting method. As the n-type doping impurityelement may be typically used phosphorus (P) or arsenic (As). In thisembodiment, the ion doping method is performed under a condition in agas flow rate of phosphine (PH₃) diluted to 5% with hydrogen of 40 seem,the dose of 2×10¹³ atoms/cm², and the accelerating voltage of 80 kV. Inthis case, the resist mask, the first conductive layer, and the secondconductive layer function as masks for the n-type doping impurityelement and the third impurity regions 313, 314, and 326 to 328 areformed. An n-type doping impurity element in the density range of 1×10²⁰to 1×10²¹ atoms/cm³ are added to the third impurity regions 313 and 314.In this embodiment, the region of same density range as the thirdimpurity region is referred to as n⁺ region.

After the resist mask is removed, the mask made from resist is formed toperform the fourth doping treatment. By the fourth doping treatment, thefourth impurity regions 318, 319, 332, and 333 and the fifth impurityregions 316, 317, 330, and 331 are formed that is the semiconductorlayer forming the semiconductor layer forming the p-channel type TFT inwhich p-type doping impurity element is added.

A p-type doping impurity element in the density range of 1×10²⁰ to1×10²¹ atoms/cm³ are added to the fourth impurity regions 318, 319, 332,and 333. Note that, in the fourth impurity regions 318, 319, 332, and333, phosphorous (P) has been added in the preceding step (n⁻ region),but the p-type doping impurity element is added at a density that is 1.5to 3 times as high as that of phosphorous. Thus, the fourth impurityregions 318, 319, 332, and 333 have a p-type conductivity. In thisembodiment, the region of same density range as the fourth impurityregion is referred to as p⁺ region.

The fifth impurity regions 316, 317, 330, and 331 are formed to overlapwith the taper portion of the second conductive layer, and added withthe p-type impurity element in the density range of 1×10¹⁸ to 1×10²⁰atoms/cm³. In this embodiment, the region of same density range as thefifth impurity region is referred to as p⁻ region.

Though the above-described steps, the impurity regions having n-type orp-type doping impurity element are formed in the respectivesemiconductor layer. The conductive layers 304 to 307 become gateelectrodes of TFT.

Next, an insulating film (not shown) that covers substantially theentire surface is formed. In this embodiment, a 50 nm thick siliconoxide film is formed by plasma CVD. Of course, the insulating film isnot limited to a silicon oxide film, and other insulating filmscontaining silicon may be used in a single layer or a laminationstructure.

Then, a step of activating the impurity element added to the respectivesemiconductor layers is conducted. In this activation step, a rapidthermal annealing (RTA) method using a lamp light source, a method ofirradiating light emitted from a YAG laser or excimer laser from theback surface, heat treatment using a furnace, or a combination thereofis employed.

Further, although an example in which the insulating film is formedbefore the activation is shown in this embodiment, a step of forming theinsulating film may be conducted after the activation is conducted.

Next, a first interlayer insulating film 308 is formed of a siliconnitride film, and heat treatment (300 to 550° C. for 1 to 12 hours) isperformed, thereby conducting a step of hydrogenating the semiconductorlayers. This step is a step of terminating dangling bonds of thesemiconductor layers by hydrogen contained in the first interlayerinsulating film 308. The semiconductor layers can be hydrogenatedirrespective of the existence of an insulating film (not shown) formedof a silicon oxide film. As another means for hydrogenation, plasmahydrogenation (using hydrogen excited by plasma) may be conducted.

Next, a second interlayer insulating film 309 is formed from an organicinsulating material on the first interlayer insulating film 308. In thisembodiment, an acrylic resin film 309 a with a thickness of 1.6 μm isformed by a coating method. Further, the silicon nitride film 309 b witha thickness of 200 nm is formed by using a sputtering method. In thisembodiment, an example of depositing the silicon nitride film on theacrylic resin film with a thickness of 1.6 μm is shown. The material orthe thickness of the insulating film are not limited. In the case that acapacity is formed between the gate electrode and the power sourcecurrent line that is formed on the gate electrode, the thickness of theorganic insulating film and the inorganic insulating film may be 0.5 μmto 2.0 μm.

Next, the pixel electrode 334 is formed that contacts to the drainregion of the current control TFT 404 including p-channel TFT to contactand overlap with the connection electrode to be formed later. In thisembodiment, the pixel electrode functions as an anode of OLED, and is atransparent conductive film to pass the light from OLED to the pixelelectrode.

The contact hole that reaches the conductive layer to be the gateelectrode or the gate wiring, and the contact hole that reach eachimpurity region. In this embodiment, the plural etching treatments areperformed sequentially. In this embodiment, the third interlayerinsulating film is etched using the second interlayer insulating film asan etching stopper, and the first interlayer insulating film is etchedafter the second interlayer insulating film is etched using the firstinterlayer insulating film as the etching stopper.

Thereafter, the electrodes 335 to 341 are formed by using Al, Ti, Mo, Wand the like. Specifically, a source wiring, a power source supply line,an extraction electrode, and a connection electrode are formed. As thematerial of the electrodes and the wirings, a lamination film having Alfilm (350 nm thickness) including Ti film (110 nm thickness) andsilicon, and Ti film (50 nm thickness) is used. And patterning isperformed. Thus, the source electrode, the source wiring, the connectionelectrode, the extraction electrode, and the power source supply lineare formed appropriately. Further, the extraction electrode forcontacting with the gate wiring overlapped with the interlayerinsulating film is provided in the edge portion of the gate wiring. Theinput-output terminal portion in which the plural electrodes forconnecting with an external circuit and an external power source isprovided are formed in other edge portions of each wiring. Theconnection electrode 341 to contact and overlap with the pixel electrode334 that is formed previously contacts with the drain region of thecurrent control TFT 404.

As described above, a driving circuit 402 having an n-channel TFT 405, ap-channel TFT 406, and a CMOS circuit that combines complementary then-channel TFT 405 and a p-channel TFT 406, and a pixel portion 401provided the plural n-channel TFTs 403 or the plural p-channel TFTs 404in one pixel are formed.

In this embodiment, the length of the channel formation region 329 ofthe p-channel TFT 404 connecting to OLED 400 is quite long. For example,the top surface structure may be formed as shown in FIG. 5. The lengthof channel L is 500 μm in FIG. 5. The width of channel W is 4 μm.

The patterning of each electrode is completed, the heat treatment isconducted removing resist. The insulators 342 a, 342 b referred to asbank are formed to overlap with the edge portion of the pixel electrode334. The bank 342 a and 342 b may be formed by using an insulating filmcontaining silicon or resin film. Here, after the bank 342 a is formedby patterning the insulating film made from an organic resin film andthe silicon nitride film is formed by the sputtering method. And thebank 342 b is formed by performing patterning.

Next, an EL layer 343 is formed on the pixel electrode 334 whose endsare covered with the banks and a cathode 344 of an OLED is formedthereon.

An EL layer 343 (a layer for light emission and for moving of carriersto cause light emission) has a light emitting layer and a freecombination of electric charge transporting layers and electric chargeinjection layers. For example, a low molecular weight organic ELmaterial or a high molecular weight organic EL material is used to forman EL layer. An EL layer may be a thin film formed of a light emittingmaterial that emits light by singlet excitation (fluorescence) (asinglet compound) or a thin film formed of a light emitting materialthat emits light by triplet excitation (phosphorescence) (a tripletcompound). Inorganic materials such as silicon carbide may be used forthe electric charge transporting layers and electric charge injectionlayers. Known organic EL materials and inorganic materials can beemployed.

It is said that the preferred material of a cathode 344 is a metalhaving a small work function (typically, a metal element belonging toGroup 1 or 2 in the periodic table) or an alloy of such metal. The lightemission efficiency is improved as the work function becomes smaller.Therefore, an alloy material containing Li (lithium) that is one ofalkali metals is particularly desirable as the cathode material. Thecathode also functions as a wiring common to all pixels and has aterminal electrode in an input terminal portion through a connectionwiring.

FIG. 7 is a state that is completed so far.

Next, the OLED having at least a cathode, an organic compound layer, andan anode is preferably sealed by an organic resin, a protective film, asealing substrate, or a sealing can to cut the OLED completely off fromthe outside and prevent permeation of external substances, such asmoisture and oxygen, that accelerate degradation due to oxidization ofthe EL layer. However, it is not necessary to provide the protectivefilm or the like in the input-output terminal portions to which an FPCneeds to be connected later.

The FPC (flexible printed circuit) is attached to the electrodes of theinput-output terminal portions using an anisotropic conductive material.The anisotropic conductive material is composed of a resin andconductive particles several tens to several hundreds μm in diameterwhose surfaces are plated by Au or the like. The conductive particleselectrically connect the electrodes of the input-output terminalportions with wirings formed in the FPC.

If necessary, an optical film such as a circularly polarizing platecomposed of a polarizing plate and a phase difference plate may beprovided and an IC chip may be mounted.

According above the steps, the module type light emitting deviceconnected FPC is completed.

Moreover, when displaying by full color, the equivalent circuit diagramin the pixel portion of this embodiment is shown in FIG. 8. A referencenumeral 701 in FIG. 8 corresponds to the switching TFT 403 of FIG. 7,and a reference numeral 702 corresponds to a current control TFT 404.The pixel to which OLED 703R which displays red light to the drainregion of the current control TFT 404 is connected, and anode side powersupply line R 706R is prepared in the source region. Moreover, thecathode side power supply line 700 is formed in OLED 703R. Moreover thepixel to which OLED 703G which displays green light to the drain regionof the current control TFT are connected, and an anode side power supplyline G 706 G are prepared in the source region. Moreover, the pixel towhich OLED 703B which displays blue light to the drain region of thecurrent control TFT is connected, and anode side power supply line B706B is prepared in the source region. Different voltage is impressed toeach pixel that has different colors according to EL material,respectively. In order to reduce the channel conductance gd, the channellength is made longer, and made to drive as an ON state with a high gatevoltage rather than conventional cases.

In this embodiment, as a display driving method, time division grayscale driving method that is a kind of line sequential driving method.For inputting an image signal to the source wiring, the both analogsignal and digital signal may be used. The driving circuit and the likemay be appropriately designed according to the image signal.

Embodiment 2

This embodiment shows a top view (FIGS. 5 and 6) that is enlarged a partof the pixel portion in Embodiment 1, and a top view that is differentin a part from FIGS. 5 and 6 is shown in FIGS. 13A and 13B.

FIG. 13A is a corresponding top view to FIG. 6, and same portionsthereof are indicated by same symbols. FIG. 13A is an example ofsemiconductor layer 1102 that has different patterning shape that isadopted instead of the semiconductor layer 102 shown in FIG. 6. In thisembodiment, the semiconductor layer 1102 is meandering. As shown in FIG.13A, channel length L×channel width W is the same as FIG. 6, and is set500 μm×4 μm. FIG. 13A is same as Embodiment 1 except the semiconductorlayer 1102 that has a different patterning shape, so that anotherexplanation may be referenced to Embodiment 1.

FIG. 13B shows another different top view. Same portion corresponding toFIG. 6 are indicated by same symbols. FIG. 13B shows a semiconductorlayer 1202 that has different patterning shape that is adopted insteadof the semiconductor layer 102 shown in FIG. 6, and an electrode 1200that is adopted instead of the electrode 100. The channel length in FIG.13B is 165 μm. FIG. 13B is same as Embodiment 1 except the semiconductorlayer 1202 and the electrode 1200 that has a different patterning shape,so that another explanation may be referenced to Embodiment 1.

This embodiment can be combined with Embodiment Mode or Embodiment 1.

Embodiment 3

The top view and cross-sectional view of the module type light emittingdevice (also referred to as EL module) obtained by Embodiment 1 or 2 areillustrated.

FIG. 6A is a top view of an EL module, and FIG. 14B is a cross-sectionalview taken along the line A-A′ of FIG. 14A. In FIG. 14A, a baseinsulating film 501 is formed on a substrate 500 (for example, a heatresistant glass), and a pixel portion 502, a source driving circuit 504,and a gate driving circuit 503 are formed thereon. These pixel portionand driving circuit may be obtained by Embodiment 1 or 2.

Reference numeral 518 is an organic resin, reference numeral 519 is aprotective film, a pixel portion and a driving circuit are covered withthe organic resin 518, and the organic resin 518 is covered with theprotective film 518. In addition, the cover material may be used to sealusing bonding material. The cover material may be bonded as a supportmedium before peeled off.

Wiring 508 for transmitting signals to be input to the source drivingcircuit 504 and the gate driving circuit 503 is provided. A videosignal, a clock signal, etc., are received through the wiring 508 from aflexible printed circuit (FPC) 509 used as an external input terminal.Although only the FPC is illustrated, a printed wiring board (PWB) maybe attached to the FPC. The light-emitting device described in thisspecification also comprises a combination of the light-emitting devicemain unit and the FPC or a PWB attached to the main unit.

The structure of this embodiment as seen in the sectional view of FIG.14B will next be described. A base insulating film 501 is provided onthe substrate 500, and the pixel portion 502 and the gate drivingcircuit 503 are formed on the insulating film 501. The pixel portion 502is constituted by current control TFTs 511 and a plurality of pixelsincluding pixel electrodes 512 electrically connected to the drains ofthe current control TFTs 511. The gate driving circuit 503 is formed byusing a CMOS circuit including a combination of an n-channel TFT 513 anda p-channel TFT 514.

TFTs in these circuits (including TFTs 511, 513, and 514) may bemanufactured in accordance with the n-channel TFT and the p-channel TFTof Embodiment 1.

Each pixel electrode 512 functions as an cathode of a light emittingelement. Banks 515 are formed at the opposite ends of the pixelelectrode 512. An organic compound layer 516 and a anode 517 of thelight emitting element are formed on the pixel electrode 512.

An organic compound layer 516 (a layer for light emission and for movingof carriers to cause light emission) has a light emitting layer and afree combination of electric charge transporting layers and electriccharge injection layers. For example, a low molecular weight organiccompound material or a high molecular weight organic compound materialis used to form an organic compound layer. An organic compound layer 516may be a thin film formed of a light emitting material that emits lightby singlet excitation (fluorescence) (a singlet compound) or a thin filmformed of a light emitting material that emits light by tripletexcitation (phosphorescence) (a triplet compound). Inorganic materialssuch as silicon carbide may be used for the electric charge transportinglayers and electric charge injection layers. Known organic materials andinorganic materials can be employed.

The anode 517 also functions as a wiring connected in common to all thepixels. The anode 517 is electrically connected to the FPC 509 viaconnection wiring 508. All the devices contained in the pixel portion502 and the gate driving circuit 503 are covered with the anode 517, theorganic resin 518 and the protective film 519.

Preferably, a material having the highest possible transparency ortranslucence for visible light is used as the sealing material 518.Also, preferably, the sealing material 518 has the highest possibleeffect of limiting permeation of water and oxygen.

It is also preferable to provide the protective film 519 formed of a DLCfilm or the like at least on the surface of the sealing material 518(exposed surface), as shown in FIGS. 14A and 14B, after thelight-emitting device has been completely covered with the sealingmaterial 518. The protective film may be provided on the entire surfaceincluding the back surface of the substrate. In such a case, care mustbe exercised to avoid forming the protective film on the region whereexternal input terminal (FPC) is provided. To avoid film forming on theexternal input terminal region, a mask may be used or the terminalregion may be covered with a tape such as a Teflon tape (registeredmark) used as a masking tape in CVD apparatus. For forming theprotective film 519, a silicon nitride film, DLC film, or AlNxOy filmmay be used.

The light emitting device is enclosed in the above-described structurewith the protective film 519 to completely isolate the light emittingdevice from the outside and to prevent substances which promotedegradation of the organic compound layer by oxidation, e.g., water andoxygen from entering the light emitting device from the outside. Thus,the light emitting device having improved reliability can be obtained.

Another arrangement is conceivable in which a pixel electrode is used asa cathode and an organic compound layer and an anode having property oftransitivity are formed in combination to emit light in a directionopposite to the direction indicated in FIG. 14. FIG. 15 shows an exampleof such an arrangement. This arrangement can be illustrated in the sametop view as FIG. 14 and will therefore be described with reference to across-sectional view only.

The structure shown in the cross-sectional view of FIG. 15 will bedescribed. An insulating film 610 is formed on a film substrate 600, anda pixel portion 602 and a gate-side drive circuit 603 are formed overthe insulating film 610. The pixel portion 602 is formed by a pluralityof pixels including a current control TFT 611 and a pixel electrode 612electrically connected to the drain of the current control TFT 611. Agate-side drive circuit 603 is formed by using a CMOS circuit having acombination of an n-channel TET 613 and a p-channel TFT 614.

These TFTs (611, 613, 614, etc.) may be fabricated in the same manner asthe n-channel TFT and the p-channel TFT of Embodiment 1.

The pixel electrode 612 functions as an anode of the light emittingelement. Banks 615 are formed at opposite ends of the pixel electrode612, and an organic compound layer 616 and a cathode 617 of the lightemitting element are formed over the pixel electrode 612.

The cathode 617 also functions as a common wiring element connected toall the pixels and is electrically connected to a FPC 609 via connectionwiring 608. All the elements included in the pixel portion 602 and thegate-side drive circuit 603 are covered with the cathode 617, an organicresin 618 and a protective film 619. A cover member 620 is bonded to theelement layer by an adhesive. A recess is formed in the cover member anda desiccant 621 is set therein.

In the arrangement shown in FIG. 15, the pixel electrode is used as theanode while the organic compound layer and the cathode are formed incombination, so that light is emitted in the direction of the arrow inFIG. 15.

While the top gate TFTs have been described by way of example, thepresent invention can be applied irrespective of the TFT structure. Forexample, the present invention can be applied to bottom gate (invertedstaggered structure) TFTs and staggered structure TFTs.

Embodiment 4

All of the electronic equipments incorporated various modules (activematrix EL module) having OLED are completed by implementing the presentinvention.

Following can be given as such electronic equipments: video cameras;digital cameras; head mounted displays (goggle type displays); carnavigation systems; projectors; car stereos; personal computers;portable information terminals (mobile computers, mobile phones orelectronic books etc.) etc. Examples of these are shown in FIGS. 16 and17.

FIG. 16A is a personal computer which comprises: a main body 2001; animage input section 2002; a display section 2003; and a keyboard 2004etc.

FIG. 16B is a video camera which comprises: a main body 2101; a displaysection 2102; a voice input section 2103; operation switches 2104; abattery 2105 and an image receiving section 2106 etc.

FIG. 16C is a mobile computer which comprises: a main body 2201; acamera section 2202; an image receiving section 2203; operation switches2204 and a display section 2205 etc.

FIG. 16D is a goggle type display which comprises: a main body 2301; adisplay section 2302; and an arm section 2303 etc.

FIG. 16E is a player using a recording medium in which a program isrecorded (hereinafter referred to as a recording medium) whichcomprises: a main body 2401; a display section 2402; a speaker section2403; a recording medium 2404; and operation switches 2405 etc. Thisapparatus uses DVD (digital versatile disc), CD, etc. for the recordingmedium, and can perform music appreciation, film appreciation, games anduse for Internet.

FIG. 16F is a digital camera which comprises: a main body 2501; adisplay section 2502; a view finder 2503; operation switches 2504; andan image receiving section (not shown in the figure) etc.

FIG. 17A is a mobile phone which comprises: a main body 2901; a voiceoutput section 2902; a voice input section 2903; a display section 2904;operation switches 2905; an antenna 2906; and an image input section(CCD, image sensor, etc.) 2907 etc.

FIG. 17B is a portable book (electronic book) which comprises: a mainbody 3001; display sections 3002 and 3003; a recording medium 3004;operation switches 3005 and an antenna 3006 etc.

FIG. 17C is a display which comprises: a main body 3101; a supportingsection 3102; and a display section 3103 etc.

In addition, the display shown in FIG. 17C has small and medium-sized orlarge-sized screen, for example a size of 5 to 20 inches. Further, tomanufacture the display part with such sizes, it is preferable tomass-produce by gang printing by using a substrate with one meter on aside.

As described above, the applicable range of the present invention isextremely large, and the invention can be applied to electronicequipments of various areas. Note that the electronic devices of thisembodiment can be achieved by utilizing any combination of constitutionsin Embodiments 1 to 3.

According to the present invention, in a pixel portion where a pluralityof TFTs are arranged, in the TFTs that supply currents to the OLED, notonly simple dispersion of on-current but also normalized dispersionthereof can be reduced, resulting in particularly reducing thedispersion of the brightness of a display device having the OLED.

Furthermore, according to the present invention, even when thedispersion in the TFT fabrication process such as illuminationconditions of the laser light or the like is caused, the dispersion ofthe electric characteristics between the TFTs can be reduced.

Still furthermore, according to the present invention, other than thereduction of the dispersion between the individual TFTs, the dispersionof the OLED itself caused by an area contraction of the EL layer due tothe patterning and the heat treatment can be reduced.

Furthermore, according to the present invention, other than thereduction of the dispersion between the individual TFTs, even when theOLED is deteriorated for some reason, the current flowing to the OLEDcan be maintained constant, resulting in maintaining a constantbrightness.

Still furthermore, according to the present invention, since part of thecapacitance C_(ox) of the TFT can be intentionally used as the retentioncapacitance, simplification of the pixel structure and an improvement inthe open area ratio can be attained.

1. A light emitting device having a light emitting element, the lightemitting element comprising: a cathode; an organic compound layer overthe cathode; and an anode over the organic compound layer; wherein athin film transistor connected to the light emitting element comprised apolysilicon film including a channel forming region interposed between aleast one pair of impurity regions, and wherein a channel length L ofthe film transistor is 100 μm or more.
 2. A light emitting device as setforth in claim 1, wherein a ratio of a channel width W of the thin filmtransistor to the channel length L thereof is from 0.1 to 0.01.
 3. Alight emitting device according to claim 1, wherein the thin filmtransistor connected to the light emitting element is a p-channel typethin film transistor or an n-channel type thin film transistor.
 4. Alight emitting device according to claim 1, wherein the thin filmtransistor connected to the light emitting element, in the range thatthe sum of source-drain voltage Vd and threshold voltage Vth is largerthan gate voltage Vg, has a channel conductance gd from 0 to 1×10⁻⁸ S.5. A light emitting device according to claim 1, wherein the thin filmtransistor connected to the light emitting element, in the range thatthe sum of source-drain voltage Vd and threshold voltage Vth is largerthan gate voltage Vg, has a channel conductance gd from 0 to 5×10⁻⁹ S.6. A light emitting device according to claim 1, wherein the lightemitting device is incorporated into an electronic equipment selectedfrom the group consisting of a personal computer, a video camera, amobile computer, a goggle type display, a recording medium, a digitalcamera, a mobile phone and a display.
 7. A light emitting deviceaccording to claim 1, further comprising: a desiccant; an organic resinover the anode; a protective film over the organic resin; and a covermember covering the protective film, wherein the desiccant is set in thecover member.
 8. A light emitting device having a light emittingelement, the light emitting element comprising: a cathode; an organiccompound layer over the cathode; and an anode over the organic compoundlayer; wherein a thin film transistor connected to the light emittingelement comprises a polysilicon film including a channel forming regioninterposed between at least one pair of impurity regions, and wherein aratio of a channel width W of the thin film transistor to the channellength L thereof is from 0.1 to 0.01.
 9. A light emitting deviceaccording to claim 8, wherein the thin film transistor connected to thelight emitting element is a p-channel type thin film transistor or ann-channel type thin film transistor.
 10. A light emitting deviceaccording to claim 8, wherein the thin film transistor connected to thelight emitting element, in the range that the sum of source-drainvoltage Vd and threshold voltage Vth is larger than gate voltage Vg, hasa channel conductance gd from 0 to 1×10⁻⁸ S.
 11. A light emitting deviceaccording to claim 8, wherein the thin film transistor connected to thelight emitting element, in the range that the sum of source-drainvoltage Vd and threshold voltage Vth is larger than gate voltage Vg, hasa channel conductance gd from 0 to 5×10⁻⁹ S.
 12. A light emitting deviceaccording to claim 8, wherein the light emitting device is incorporatedinto an electronic equipment selected from the group consisting of apersonal computer, a video camera, a mobile computer, a goggle typedisplay, a recording medium, a digital camera, a mobile phone and adisplay.
 13. A light emitting device according to claim 8, furthercomprising: a desiccant; an organic resin over the anode; a protectivefilm over the organic resin; and a cover member covering the protectivefilm, wherein the desiccant is set in the cover member.
 14. A lightemitting device having a light emitting element, the light emittingelement comprising: a cathode; an organic compound layer over thecathode; and an anode over the organic compound layer; wherein a thinfilm transistor connected to the light emitting element comprises apolysilicon film including a channel forming region interposed between asource region and a drain region, and wherein a thin film transistor, inthe range that the sum of source-drain voltage Vd and threshold voltageVth is larger than gate voltage Vg, has a channel conductance gd from 0to 2×10⁻⁹ S.
 15. A light emitting device according to claim 14, whereinthe thin film transistor connected to the light emitting element is ap-channel type thin film transistor or an n-channel type thin filmtransistor.
 16. A light emitting device according to claim 14, whereinthe thin film transistor connected to the light emitting element, in therange that the sum of source-drain voltage Vd and threshold voltage Vthis larger than gate voltage Vg, has a channel conductance gd from 0 to1×10⁻⁸ S.
 17. A light emitting device according to claim 14, wherein thethin film transistor connected to the light emitting element, in therange that the sum of source-drain voltage Vd and threshold voltage Vthis larger than gate voltage Vg, has a channel conductance gd from 0 to5×10⁻⁹ S.
 18. A light emitting device according to claim 14, wherein thelight emitting device is incorporated into an electronic equipmentselected from the group consisting of a personal computer, a videocamera, a mobile computer, a goggle type display, a recording medium, adigital camera, a mobile phone and a display.
 19. A light emittingdevice according to claim 14, further comprising: a desiccant; anorganic resin over the anode; a protective film over the organic resin;and a cover member covering the protective film, wherein the desiccantis set in the cover member.
 20. A light emitting device comprising: alight emitting element having a cathode, an organic compound layer overthe cathode and an anode over the organic compound layer; and a thinfilm transistor connected to the light emitting element, wherein thethin film transistor comprises: a polysilicon film including arectangular-shaped channel forming region interposed between at leastone pair of impurity regions; a gate insulating film formed over therectangular-shaped channel region; and a gate electrode formed over thegate insulating film, wherein a channel length L of the rectangle-shapedchannel region is 100 μm or more.
 21. A light emitting device accordingto claim 20, wherein the thin film transistor connected to the lightemitting element is a p-channel type thin film transistor or ann-channel type thin film transistor.
 22. A light emitting deviceaccording to claim 20, wherein the thin film transistor connected to thelight emitting element, in the range that the sum of source-drainvoltage Vd and threshold voltage Vth is larger than gate voltage Vg, hasa channel conductance gd from 0 to 1×10⁻⁸ S.
 23. A light emitting deviceaccording to claim 20, wherein the thin film transistor connected to thelight emitting element, in the range that the sum of source-drainvoltage Vd and threshold voltage Vth is larger than gate voltage Vg, hasa channel conductance gd from 0 to 5×10⁻⁹ S.
 24. A light emitting deviceaccording to claim 20, wherein the light emitting device is incorporatedinto an electronic equipment selected from the group consisting of apersonal computer, a video camera, a mobile computer, a goggle typedisplay, a recording medium, a digital camera, a mobile phone and adisplay.
 25. A light emitting device according to claim 20, furthercomprising: a desiccant; an organic resin over the anode; a protectivefilm over the organic resin; and a cover member covering the protectivefilm, wherein the desiccant is set in the cover member.
 26. A lightemitting device comprising: a light emitting element including acathode, an organic compound layer over the cathode and an anode overthe organic compound layer; a thin film transistor connected to thelight emitting element, wherein the thin film transistor comprises: apolysilicon film including a rectangle-shaped channel forming regioninterposed between at least one pair of impurity regions; a gateinsulating film formed over the rectangular-shaped channel region; and agate electrode formed over the gate insulating film, wherein a ratio ofa channel width W of the rectangular-shaped channel region to a channellength L of the rectangular-shaped channel region is from 0.1 to 0.01.27. A light emitting device according to claim 26, wherein the thin filmtransistor connected to the light emitting element is a p-channel typethin film transistor or an n-channel type thin film transistor.
 28. Alight emitting device according to claim 26, wherein the thin filmtransistor connected to the light emitting element, in the range thatthe sum or source-drain voltage Vd and threshold voltage Vth is largerthan gate voltage Vg, has a channel conductance gd from 0 to 1×10⁻⁸ S.29. A light emitting device according to claim 26, wherein the thin filmtransistor connected to the light emitting element, in the range thatthe sum of source-drain voltage Vd and threshold voltage Vth is largerthan gate voltage Vg, has a channel conductance gd from 0 to 5×10⁻⁹ S.30. A light emitting device according to claim 26, wherein the lightemitting device is incorporated into an electronic equipment selectedfrom the group consisting of a personal computer, a video camera, amobile computer, a goggle type display, a recording medium, a digitalcamera, a mobile phone and a display.
 31. A light emitting deviceaccording to claim 26, further comprising: a desiccant; an organic resinover the anode; a protective film over the organic resin; and a covermember covering the protective film, wherein the desiccant is set in thecover member.
 32. A light emitting device comprising: a light emittingelement including a cathode, an organic compound layer over the cathodeand an anode over the organic compound layer; a thin film transistorconnected to the light emitting element, wherein the thin filmtransistor comprises: a polysilicon film including a rectangular-shapedchannel forming region interposed between at least one pair of impurityregions; a gate insulating film formed over the rectangular-shapedchannel region; and a gate electrode formed over the gate insulatingfilm, wherein the thin film transistor has a channel conductance gd from0 to 2×10⁻⁹ S in the range that the sum of source-drain voltage Vd andthreshold voltage Vth is larger than gate voltage Vg.
 33. A lightemitting device according to claim 32, wherein the thin film transistorconnected to the light emitting element is a p-channel type thin filmtransistor or an n-channel type thin film transistor.
 34. A lightemitting device according to claim 32, wherein the thin film transistorconnected to the light emitting element, in the range that the sum ofsource-drain voltage Vd and threshold voltage Vth is larger than gatevoltage Vg, has a channel conductance gd from 0 to 1×10 ⁻⁸ S.
 35. Alight emitting device according to claim 32, wherein the thin filmtransistor connected to the light emitting element, in the range thatthe sum of source-drain voltage Vd and threshold voltage Vth is largerthan gate voltage Vg, has a channel conductance gd from 0 to 5×10⁻⁹ S.36. A light emitting device according to claim 32, wherein the lightemitting device is incorporated into an electronic equipment selectedfrom the group consisting of a personal computer, a video camera, amobile computer, a goggle type display, a recording medium, a digitalcamera, a mobile phone and a display.
 37. A light emitting deviceaccording to claim 32, further comprising: a desiccant; an organic resinover the anode; a protective film over the organic resin; and a covermember covering the protective film, wherein the desiccant is set in thecover member.